Remove code using device mapper.

(Closes: #860833, #853927, #853163)

This code doesn't work in d-i and it has some issues outside d-i. All
architectures can use the grub-mount codepath, which was already the
default anyway.

This also removes the dependency on dmsetup.

Signed-off-by: Ivo De Decker <ivodd@debian.org>